1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to measuring the quality of wireless channels, and more particularly, to measuring the quality of wireless channels by measuring contentions between stations in a wireless network employing a carrier sense multiple access/collision avoidance (CSMA/CA) medium access control (MAC) method specified in the IEEE 802.11 standard.
2. Description of the Related Art
To improve the performance of apparatuses for transmitting wireless data, it is necessary to measure the quality of wireless channels that transmit/receive data frames. The measured quality of wireless channels can be a standard index for providing users with high quality services. For example, in a plurality of wireless channels, when a current wireless channel is of bad quality, the best quality wireless channel is selected by measuring the quality of the wireless channels. The quality of wireless channels is influenced by the strength of radio signals, contentions caused by a plurality of wireless stations using a common channel, etc.
To effectively use the function of a network in transmitting/receiving data via wired or wireless networks, a standard communication specification regarding what, how, and when to communicate between objects is provided, which is called a protocol. A communication protocol based on an open systems interconnection (OSI) 7-layer structure is determined according to the characteristic of a network, such as whether the protocol is used for wired or wireless networks, what transfer medium is used to transmit data, etc.
When the OSI 7-layer uses a wireless local area network (LAN) as a transfer medium, a data link layer and a physical layer perform a function of the wireless LAN as specified in wireless LAN standards. The data link layer mainly controls wireless link errors, access of nodes to a wireless network, etc. The physical layer mainly transmits wireless packets via wireless frequencies. Data link layer can be classified as logical link control layers and MAC layers. The logical link control layer synchronizes an OSI layer 2 and controls errors of the layer. The MAC layer controls access of neighboring nodes and a wireless medium so that a plurality of nodes can effectively use the transfer medium without collision. The IEEE 802.11 wireless LAN standards specify the function of the MAC layer and the physical layer.
MAC can be a contention-free method that divides channels into a frequency division multiple access (FDMA) channel or a time division multiple access (TDMA) channel based on time and frequency bands and maps a specific channel to be used by a specific user only, or a contention method in which a medium is shared by many users, access is allowed to the medium whenever users transmit data, and, if a contention occurs, the contention is resolved using an algorithm. Carrier sense multiple access/collision avoidance (CSMA/CA) is a representative contention-based MAC protocol in which a carrier sensing scheme is used to check a transfer medium and control access to the transfer medium so that a transmission node does not waste transmission capacity caused by collisions of frames of data by avoiding the collisions. The CSMA/CA method is used as the MAC standard in the IEEE 802.11.
FIG. 1 illustrates the CSMA/CA method using a random backoff process. Referring to FIG. 1, distribute inter-frame space (DIFS) indicates spaces between frames to protect frame transmission.
A first station that wishes to transmit frames senses the status of a channel. If the channel is idle, the first station transmits the frames. Otherwise, if the channel is sensed busy, the first station waits for a period of time before it transmits the frames (Defer Access), which is a backoff process. The period of time during which the transmission of frames is detected is randomly determined just before proceeding to a backoff process, which is a backoff time. When other stations finish using the channel, it is checked that the channel is idle during the DIFS time. To avoid collision with a second station that waits for the transmission of frames, a backoff timer is set equal to the backoff time, and, if the backoff timer is reduced to 0, the second station can transmit the frames.
The backoff time may not fully elapse before a station transmits the frames but a period of time in which the channel is sensed busy is used during the backoff process. The station initializes the backoff timer to the backoff time when entering the backoff process, reduces the backoff timer only while the channel is idle, and, if the backoff timer is reduced to 0, transmits the frames. Therefore, the station that is reducing the backoff timer when the channel is idle, if the channel is busy, stops the backoff timer, waits for a period of time, and, if the channel is idle again, transmits the frames. In the CSMA/CA method, a control packet such as Request to Send/Clear to Send (RTS/CTS) is used to determine when a station uses the channel and prevent other stations from using the transfer medium for a period of time, thereby reducing possibilities of collisions in wireless channels.
FIG. 2 illustrates transmissions of frames when a CSMA/CA based contention occurs, and also illustrates how a plurality of stations resolve the contention and transmit frames using a CSMA/CA based random backoff process.
Referring to FIG. 2, stations B, C, and D check whether a channel is idle or busy when they have a frame to transmit. Since the channel is busy when a station A transmits a frame, stations B, C, and D do not transmit their frames immediately and enter a backoff process. During the backoff process, stations B, C and D set a backoff time randomly, wait until the channel is idle, operate the backoff timer, and reduce the backoff time. After station A finishes the transmission of the frame, stations B, C, and D remain under the backoff process. Because station C had the shortest backoff time, it occupies the channel first and transmits its frame.
Once station C occupies the channel, other stations determine that the channel is busy. Stations B and C that were operating their respective backoff timers stop the backoff timers and wait until the channel is idle again. While station C transmits the frame, a station E that has a frame to transmit cannot transmit the frame since the channel is busy and enters the backoff process as the stations B and D do so. By continuously performing these operations, station B transmits the frame when its backoff timer is reduced to 0 when the channel has an idle status.
In the CSMA/CA method, a station confirms whether it succeeds in transmitting a frame by receiving an acknowledgement (ACK) frame. If the station fails to receive the ACK frame within a designated time, the station decides that a channel error occurred or channels collided during the transmission of the frame and transmits the frame again through the backoff process. The number of attempts a station will make to transmit a frame is restricted. If the station fails to transmit the frame within the restricted number of transmission attempts, the station gives up transmitting the frame.
The CSMA/CA MAC method aims at effectively resolving contentions since a specific station does not use a specific channel such as TDMA or FDMA but a plurality of stations use a common channel. A contention based CSMA/CA MAC method is affected by link quality, which is lowered due to insecurity of signals such as noise or interference occurring in a physical channel, and contentions between stations. Therefore, systems employing the CSMA/CA MAC method must focus on the contention between stations when measuring the quality of wireless channels such as link throughput.